Microporous membranes are widely used in various fields including medical dialysis, environmental filtration, food purification and so on. Recently, microporous membranes are being utilized as a separator for a lithium secondary battery (e.g. lithium ion battery, lithium polymer battery, etc.). In particular, microporous membranes adapted for a lithium polymer battery play a role not only as cathode and anode separators but also as an ion conductive medium, namely, an electrolyte. The microporous membrane used as the separator and the electrolyte for a battery may be mainly produced from polyolefinic resin. In the case where a polyolefinic resin having high crystallinity, such as polyethylene and polypropylene, is used as a separator for a lithium secondary battery, tensile strength, rigidity, and impact strength of the separator may be enhanced, and as well ion permeability may be greatly increased.
The production of the microporous membrane from the polyolefinic resin may include the use of a precursor film. The production of the microporous membrane using a film may include for example MCS (Melt Casting and Stretching: 1 phase), thermally-induced phase separation (2 phase), and phase inversion (3 phase). Particularly useful are MSS using only a polymer without the use of a solvent based on a dry process, and thermally-induced phase separation essentially using a polymer, a solvent and an extracting agent based on a wet process.
In the dry process, dry production (hereinafter, referred to as “uniaxial dry stretching”) for forming micropores through uniaxial stretching is very economical because the production process is simple and thus mass production is possible, and is advantageous because it may enhance tensile strength in a machine direction (MD), and is environmentally friendly thanks to an organic solvent not being used. However, this method is disadvantageous because tensile strength in a transverse direction (TD) is low due to unidirectional orientation of polymer chains through uniaxial stretching.
On the other hand, the wet process for producing a microporous composite membrane using a solvent causes environmental problems due to the use of the organic solvent. Furthermore, the wet process is problematic because high ion conductivity cannot be expected attributable to the remainder of the solvent used for phase separation and the limitation in pore size determined thereby.
Thus, there is required a microporous composite membrane which is manufactured using a method such as uniaxial dry stretching that is environmentally friendly and economical, and also which satisfies both mechanical strength and ion conductivity like a triple membrane structure using a first porous polymer, a second gelling polymer and a plasticizer.